1. Field of the Invention
The present invention relates to a means for holding a stamper plate in a molding metal die for injection molding optical disc substrates such CD-ROMs and the like using the stamper plate.
2. Description of the Related Art
There is known an optical disc substrate injection molding metal die for moving a stamper plate press bush using cam shaft. Refer to, for example, Japanese Patent Publication Laid-Open No. 2-295726.
FIG. 5 is a cross sectional view showing an injection molding state of a conventional optical disc substrate injection molding metal die for moving a stamper plate press bush using a cam shaft.
FIG. 6 is a schematic cross sectional view showing a state that a stamper plate press bush is moved by a conventional apparatus. A center hole is defined at the center of a disc-shaped stamper plate 1. A stamper plate mounting bush 2 includes a barrel portion 2a having an outside diameter slightly smaller than the center hole of the stamper plate 1 and a flange 3 having an outside diameter larger than the center hole of the stamper plate 1 at the lower end thereof. Further, a cutout 9 is defined to at the upper end of the stamper plate mounting bush 2. A fixed side plate 6 is provided with a fixed side disc cavity plate 4 and a hole 5 for receiving the bush 2 is defined at the center of the fixed side disc cavity plate 4. The fixed side plate 6 rotatively supports two engaging rods 7 which are engaged with the cutout 9 of the bush 2. The two engaging rods 7, 7 include planes 8, 8 each having a semicircular cross section and are coupled with each other so that they are rotated in synchronism. A spool bush 10 is inserted into the center hole of the bush 2 and has an injection hole 11 for a molten resin defined at the center thereof. A ring-shaped cooling water passage 12 is disposed to the fixed side disc cavity plate 4 in a state that it is in contact with the fixed side plate 6. A movable side disc cavity plate 17 is fixed to a movable side plate 19. A punch 18 is supported at the center of the movable side disc cavity plate 17 so as to move upward/downward in the drawing so that the punch 18 punches out a center hole of a disc substrate.
The stamper plate 1 is put into the bush 2 when an injection molding is carried out so that the outer periphery of the hole of the stamper plate is pressed by the flange 3 of the bush 2. As shown in FIG. 5, the bush 2 is engaged with the planes 8, 8 of the semi-circular cross sections of the two engaging rods 7, 7 and pressed thereby in injection molding. Next, steps of procedure for removing the stamper plate 1 will be described with reference to FIG. 6. A disc substrate molded member 20 is removed by opening the die. As shown in FIG. 6, when the two engaging rods 7 are turned in the direction of an arrow in the drawing, the planes 8 of the engaging rods 7 are disengaged once from the cutout 9 of the stamper plate mounting bush 2 and pushes the upper surface of the bush 2 in the drawing. With this operation, the stamper plate mounting bush 2 is pushed out from the center hole 5 of the fixed side disc cavity plate 4 as shown by the drawing. Thus, a worker can take out the stamper plate 1 by hand. This conventional example is a system for mechanically mounting and dismounting the stamper plate 1 by the stamper plate mounting bush 2 and can securely mount the stamper plate. However, since the example uses the two rods and a device for driving them is needed, the example has the following problems.
That is, the use of the two rods and the provision of the device for driving the rods in relation to the metal die prevent the reduction of size and weight of the die. Further, the provision of the rods and the driving device thereof in relation to the die restricts the design of the cooling water passage. Further, a certain degree of skill is required to mount and dismount the stamper plate.
FIG. 3 is a schematic cross sectional view showing another conventional example of an optical disc substrate injection molding metal die. The conventional apparatus (for example, Japanese Patent Publication No. 2-60502) is arranged to support the stamper plate using a vacuum unit.
A spool bush 30 is disposed at the center of a disc side cavity plate 29. A stamper plate 25 is supported on a stamper plate side disc cavity plate 21 as described later. The stamper plate side disc cavity plate 21 has concentric circular grooves 22, 23 defined at the center and around the outer peripheral portion thereof. A cylindrical bush 26 is disposed at the center of the stamper plate side disc cavity plate 21 and a punch 28 is provided with the cylindrical bush 26 to punch out the center hole of a disc substrate molded body 20. The cylindrical bush 26 has a peripheral portion 27 defined at the extreme end thereof, the peripheral portion 27 being inserted into the center hole of the stamper plate 25.
The aforesaid concentric circular grooves 22, 23 defined at the center and around the outer peripheral portion of the stamper plate side disc cavity plate 21 are coupled with a not shown vacuum pump through an communicating hole 24 to vacuum one surface of the stamper plate 25 onto the stamper plate side disc cavity plate 21 to thereby support the stamper plate 25 on the cavity plate 21. This conventional apparatus has an advantage that the stamper plate 25 can be simply removed from the disc cavity plate 21 by stopping vacuuming. However, this conventional apparatus has the following problem. That is, the cavity of the disc substrate molding metal die must be vacuumed to provide the density of a molten resin filled into the cavity with improved uniformity. In this case, since a suction force acting on the rear surface of the stamper plate 25 is cancelled and does not function, there is a possibility that the stamper plate 25 is removed from the cylindrical bush 26 by the vibration and impact of a molding machine operating at a high speed. Further, the stamper plate 25 must be vacuumed and supported by driving the vacuum pump so far as it is mounted even if a molding operation is interrupted.
FIG. 4 is a schematic cross sectional view showing a further conventional example of an optical disc substrate injection molding metal die. The conventional example (for example, Japanese Patent Publication Laid Open No. 5-185475) supports a stamper plate using an electromagnet. A cylindrical bush 34 is disposed at the center of a disc cavity plate 31 on the side of the stamper plate and a nozzle bush 35 is disposed at the center thereof. A magnet or electromagnet 32 is disposed to the disc cavity plate 31. A punch 37 for punching out the center hole of the stamper plate 33 is provided with the other disc cavity plate 36.
This conventional example is arranged such that the stamper plate 33 is magnetically attracted by the disc-shaped electromagnet or magnet 32 disposed on the stamper plate side disc cavity plate 31 making use of that the stamper plate is composed of nickel as a ferromagnetic substance. However, this conventional example has the following problem. First, since this type of the molding die is subjected to a temperature variation up to 200.degree. C., a high pressure sliding friction force is caused between the front surface of the disc cavity plate and the rear surface of the stamper plate by the expansion and contraction of the stamper plate resulting from the temperature variation. Thus, the occurrence of scratches produced on the rear surface of the stamper plate must be prevented. Further, the corrosion of the surface of the disc cavity plate caused by a gas generated from a resin material must be prevented. To cope with this problem, an ordinary disc substrate molding metal die apparatus employs martensitic stainless steel as a material of its disc cavity plate, subjects the stainless steel to a heat treatment to provide it with a hardness of H.sub.R C 55-60 and further subjects the resulting stainless steel to a corrosion resistant surface hardening treatment such as the coating of a Ti-N hardening film having a hardness of H.sub.V 2000 with the surface roughness of the steel finished to 10 nanometers or less. In the disc substrate molding metal die apparatus shown in the above conventional example, a magnet which is much softer than that used in a conventional metal die must be buried to the surface of the disc cavity plate. The occurrence of the high pressure sliding friction force between the front surface of the disc cavity plate and the rear surface of the stamper plate cannot be prevented on the surface of the magnet.
Further, unless the surface of the disc cavity plate of the substrate molding metal die is cooled without causing a variation at an equal speed, there is a possibility that the quality of a molded disc substrate is damaged. With respect to this point, uniformity of cooling also becomes a problem in a conventional substrate molding metal die using the above monoblock cavity plate whose surface configuration is relatively uniform.
Therefore, when a foreign body such as a magnet or the like is interposed between a cooling water passage disposed on the rear surface of a disc cavity plate and the front surface of thereof as the case of the conventional example, it is very difficult to analyze a thermal distribution, by which a design is made difficult.
Even if it is supposed that the above difficulty can be solved, the above reference does not explain how the stamper plate is removed. Since it is difficult for a worker to exfoliate the stamper plate without damaging it, it is contemplated that any means for exfoliating the stamper plate without damaging it must be developed in relation to the above metal die apparatus.
An object of the present invention is to provide a means for holding a stamper plate in a molding metal die in such a manner that the stamper plate can be easily mounted to and dismounted from the surface of a disc cavity plate.